Fixes#687.
If "-" is passed as a filename, decompress data from stdin.
Currently `--format` must be passed as well, but as a next step,
we could try to infer the format from magic numbers.
As stdin is not connected to the terminal, we cannot prompt for Y/N
when warning about decompression in memory, for e.g. zip. Just default
to No, and require passing "-y" in these cases.
For zip, we have to buffer the whole stream in memory to seek into it,
just as we do with a chained decoder like `.zip.bz`.
The rar format requires an actual file (not an `impl Read`), so
we write a temp file that it can decode.
When decoding a single-file archive (e.g. file.bz), the output filename
is just `-`, since we don't know the original filename. I had to add
a bit of a hack to the tests to work around this. Another option
would be to interpret "-d" as a destination filename in this case.
When decoding a multi-file archive, I decided to unpack directly into
the destination directory, as this seemed like a better experience than
adding a top-level "-" folder inside the destination.
